After the initial excitement of being able to produce high temperature superconductors, namely materials which are superconducting above the vaporization temperature of nitrogen, the problems of producing these materials in useful form have become only too evident. Among the cuprate compositions which are particularly interesting because of their high superconducting transition temperature are the thallium compounds. These compounds are particularly difficult to prepare because of the nature of thallium oxides. Tl.sub.2 O.sub.3 is unstable, so that at the elevated processing temperatures normally employed, it decomposes to Tl.sub.2 O and O.sub.2. In order to maintain the thallium present in the oxide mixture used to form the superconductor, it is necessary to control the amount of thallium in the vapor phase and in the liquid phase of the oxide compositions. Among the other difficulties with processing thallium is that thallium is highly reactive, so that reactors which are employed must take into account the reaction of the structural materials with thallium. One is therefore confronted with working with a highly reactive material which can exist in both the vapor and liquid phases simultaneously at elevated temperatures, while trying to control the distribution of the thallium between the liquid and vapor phases in order to obtain the appropriate compositions for a high temperature superconductor.
For many applications, one wishes to have a thin high temperature superconducting film on a substrate. Among the substrates are magnesium oxide, lanthanum aluminate, yttria stabilized zirconia and sapphire. For microwave device development, sapphire has many advantages including extremely low loss tangent at low temperature, availability in large area substrates, low cost and general acceptance as a microwave substrate. In addition, for low loss films on sapphire, several orders of magnitude improvement in the Q of a microwave device can still be achieved as high temperature superconducting films are improved. However, formation of thallium high temperature superconducting films on sapphire are subject to reaction and formation of barium aluminate compounds as second phases.
There is substantial interest in being able to produce thallium cuprate high temperature superconducting films on a wide variety of substrates for production of microwave and millimeter wave applications. It is therefore of interest to provide processes and reactors which will allow for the controlled and reproducible production of high temperature superconducting films on substrates of interest for the production of devices.
The earliest attempts to manufacture thallium containing high temperature superconductors focused on bulk formation as opposed to thin film formation. Recognizing volatility of thallium, some workers opted to enclose the amorphous precursor deposit and substrate within a sealed vessel to reduce thallium loss. For example, Engler et. al., U.S. Pat. No. 4,870,052 recites the formation of a bulk thallium containing superconductor by the mixing together of oxides and heating in a preheated oven in a closed vessel in the presence of oxygen for from 1 to 5 hours at a temperature from 850.degree. to 900.degree. C. Engler most prefers the closed vessel be a sealed quartz vessel. The sample of admixed metal oxides may be held in a crucible, made for example from gold, silver, platinum, aluminum oxide or zirconium oxide and sealed inside the quartz vessel. Engler notes that even when the reaction is carried out in a sealed vessel, approximately 20% of the thallium is lost due to the volatilization and reaction with the quartz. Similarly, Gopalakrishnan, U.S. Pat. No. 4,929,594 suggests heat treatment of the mixed reactants in a tube made of non-reacting metal such as gold where the tube is welded shut. Alternatively, other workers have followed a post-composition heat treatment process in which there is no containment of volatile species. For example, Hermann et al, U.S. Pat. No. 4,962,083 discloses a preparation technique in which the mixed oxides are pressed into a pellet which is placed in a preheated tube furnace, having oxygen flowing therethrough. Studies have been reported in which the high-temperature heat treatment has been conducted under a variety of conditions, including use of an opened crucible versus a covered or sealed crucible. See e.g., Lee et. al., "Superconducting Tl-Ca-Ba-Cu-O Thin Films With Zero Resistance at Temperatures of Up to 120K", Appl. Phys. Lett., 53 (4), Jul. 25, 1988, p.329-331.
The earliest high temperature superconducting films formed were relatively small, having diameters often well less than 1 centimeter. It is highly desirable to make larger area films for many applications. Current commercially available substrates are larger than 1 centimeter, some such as lanthanum aluminate being available in up to 2 inch diameters. It is desirable to be able to use the largest available substrates to form high temperature superconducting films.
It is further desirable to manufacture double-sided films, that is, a substrate having superconductive material on multiple surfaces. Formation of such films has been extremely difficult, since processing techniques used to perfect the superconducting properties for one side of the film have damaged the superconducting properties of the other side of the film. Previously, it has been virtually impossible to repeatably form commercial grade double-sided high temperature superconducting films.